R.P. Pompe van Meerdervoort

2013 The effect of axial and transverse loading on the transport properties of ITER Nb3Sn strands

The differences in thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER), in combination with electromagnetic charging, cause axial, transverse contact and bending strains in the Nb3Sn filaments. These local loads cause distributed strain alterations, reducing the superconducting transport properties. The sensitivity of ITER strands to different strain loads is experimentally explored with dedicated probes. The starting point of the characterization is measurement of the critical current under axial compressive and tensile strain, determining the strain sensitivity and the irreversibility limit corresponding to the initiation of cracks in the Nb3Sn filaments for axial strain. The influence of spatial periodic bending and contact load is evaluated by using a wavelength of 5?mm. The strand axial tensile stress?strain characteristic is measured for comparison of the axial stiffness of the strands. Cyclic loading is applied for transverse loads following the evolution of the critical current, n-value and deformation. This involves a component representing a permanent (plastic) change and as well as a factor revealing reversible (elastic) behavior as a function of the applied load.The experimental results enable discrimination in performance reduction per specific load type and per strand type, which is in general different for each manufacturer involved. Metallographic filament fracture studies are compared to electromagnetic and mechanical load test results. A detailed multifilament strand model is applied to analyze the quantitative impact of strain sensitivity, intrastrand resistances and filament crack density on the performance reduction of strands and full-size ITER CICCs. Although a full-size conductor test is used for qualification of a strand manufacturer, the results presented here are part of the ITER strand verification program. In this paper, we present an overview of the results and comparisons.

Interstrand Resistance Measurements on the Conductor Terminations of TFPRO2 and JATF3 SULTAN Samples

A number of ITER TF cable-in-conduit samples that have been tested in the SULTAN facility performed significantly below the single strand expectations. Although performance degradation related to transverse electromagnetic loads plays a role in Nb3Sn CICCs, a similar effect can occur when the current in the sample is non-uniformly distributed among the strands, driving part of the strands into saturation already at low currents and temperatures. A sufficiently homogeneous current distribution at low electric field level (less than 10 ¿V/m) requires low interstrand resistances in the sample terminations and/or a homogeneous distribution of contact resistances between strands and the joints copper sleeve. To evaluate the influence of the joint design on qualification testing, we performed post-mortem interstrand resistance measurements on the conductor terminations of two different joint design concepts: the TFPRO2 and JATF3 SULTAN samples. The interstrand resistances and their distribution were determined on a large number of strand combinations. Accordingly, comparisons were made on how the sample performed in SULTAN. From the different design aspects, full solder filling of the terminations appears to have the strongest effect on the interstrand resistance reduction.

Spatial Periodic and Homogeneous Transverse Stress Loading on ITER TF

The transport properties of the superconducting Nb3Sn layers in the strands strongly depend on the strain state. Knowledge of the influence of axial strain, periodic bending and contact stress on the critical current (Ic) of the used strands is inevitable to gain sufficient confidence in an economic design and stable operation of ITER Nb3Sn. In the past years we have measured the Ic and n-value of various ITER strands with different layout in the TARSIS facility, when subjected to spatial periodic contact stress at a temperature of 4.2 K and in a magnet field of 12 T. Recently we have made the setup suitable for application of homogeneous load along the length of the wire (125 mm) in order to evaluate possible differences related to spatial stress and possible current distribution. We present an overview of the results obtained so far on an ITER TF bronze and internal tin strand.

{\rm Nb}_{3}{\rm Sn}

For the ITER Central Solenoid (CS), with Nb3Sn CICCs that operate under fast ramping conditions, the selection of the twist pitch lengths can have a significant impact on the performance. The critical current and temperature margin are influenced by the thermal contraction of the composite materials, the transverse electromagnetic forces, and coupling currents. The numerical cable model JackPot-ACDC is developed to calculate the interstrand coupling loss for any time-dependent current and magnet field for all strand trajectories in a CICC. It was a priori predicted that the amount of coupling loss and critical current degradation is subject to interference due to different subcable twist pitches. Here test results are discussed of the ITER CS conductor sample, manufactured according to the proposed design, optimizing the transverse load degradation, the temperature margin, and the coupling loss.

Bronze and Internal Tin Strand

For the ITER Central Solenoid (CS), with Nb3Sn CICCs that operate under fast ramping conditions, the selection of the twist pitch lengths can have a significant impact on the performance. The critical current and temperature margin are influenced by the thermal contraction of the composite materials, the transverse electromagnetic forces, and coupling currents. The numerical cable model JackPot-ACDC is developed to calculate the interstrand coupling loss for any time-dependent current and magnet field for all strand trajectories in a CICC. It was a priori predicted that the amount of coupling loss and critical current degradation is subject to interference due to different subcable twist pitches. Here test results are discussed of the ITER CS conductor sample, manufactured according to the proposed design, optimizing the transverse load degradation, the temperature margin, and the coupling loss.